Information can be found here: https://www.minecraftforum.net/forums/minecraft-java-edition/redstone-discussion-and/2933632-1-13-2-bug-items-in-hoppers-giving-off-different
basically, some items when placed in a hopper are giving off different redstone power levels when a comparator is facing out of the hopper.
A minecart by itself in a hopper seems to be giving me a power of 3. This is abnormal.
A sign and 19 other blocks is giving me a power of 2. This is abnormal.
cobblestone and 21 other blocks is giving me a power of 1. This is normal.
There are are more items giving off something similar. Some are normal, some are abnormal, some are very abnormal.
Since minecarts only stack to 1, they count as a full stack to the comparator. Same principle applies to signs; each sign counts as 4 items since they stack to 16.